Fix dark-mode email readability, silent reply failures, and add an IMAP connection watchdog

Dark theme: inbound email HTML intentionally keeps its own inline colors
for fidelity (sanitize-html.ts), but a sender's hardcoded dark text (the
common case) was unreadable once the surrounding chrome went dark. Now
forced to the theme's own text color in dark mode only; light mode keeps
full fidelity as before.

Send button: deliverAgentMessage awaited the outbound email/Telegram send
*before* recording the reply, so a broken mail/bot account (e.g. bad SMTP
creds) made the reply vanish entirely — not sent, not saved, no error
shown — with the API's catch-all masking the real cause as a generic
"Ticket not found". Channel delivery is now best-effort: the reply is
always recorded even if outbound delivery fails, and real errors surface
to the compose box instead of failing silently.

IMAP watchdog: a live IDLE connection has gone quietly unresponsive in
production more than once, with mail landing server-side but never
noticed here and no error ever firing. Added a periodic forced NOOP
round-trip with its own deadline, independent of ImapFlow's internal
socket timeout, that tears down and reconnects a connection that stops
answering.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

src/app/(admin)/tickets/[id]/ticket-thread.tsx

@@ -37,6 +37,7 @@ export function TicketThread({
   const [visibility, setVisibility] = useState<"public" | "internal">("public");
   const [pendingFile, setPendingFile] = useState<File | null>(null);
   const [sending, setSending] = useState(false);
+  const [sendError, setSendError] = useState<string | null>(null);
   const [cannedResponses, setCannedResponses] = useState<{ id: string; title: string; body: string }[]>([]);
   const [cannedOpen, setCannedOpen] = useState(false);
   const bottomRef = useRef<HTMLDivElement>(null);
@@ -82,6 +83,7 @@ export function TicketThread({
   async function sendReply() {
     if (!draft.trim() && !pendingFile) return;
     setSending(true);
+    setSendError(null);
 
     let res: Response;
     if (pendingFile) {
@@ -103,6 +105,9 @@ export function TicketThread({
       setDraft("");
       setPendingFile(null);
       if (fileInputRef.current) fileInputRef.current.value = "";
+    } else {
+      const data = await res.json().catch(() => null);
+      setSendError(data?.error ?? "Не удалось отправить сообщение");
     }
   }
 
@@ -210,6 +215,7 @@ export function TicketThread({
             </button>
           </div>
         )}
+        {sendError && <p className="mb-1.5 text-xs text-danger-soft-text">{sendError}</p>}
         {pendingFile && (
           <div className="mb-1.5 flex items-center gap-2 rounded-md border border-border bg-surface-hover px-2.5 py-1.5 text-xs">
             <Paperclip size={12} />